Khmer is the main language of Cambodia.
It is a word that is best described as
the definition of all things related to Cambodia including products, people, the country itself, style, etc.
So one could say "Khmer people" or "Khmer is next to Thailand." or "This is a Khmer dress."
The word "Khmer" and "Cambodian" are the same thing. So you can call the people of Cambodia either one.
Note that Khmer isn't a racist term in anyway. It is the same
equivalent of calling
Australians "Aussies"